The HTTP response status code 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the client must first authenticate itself with the proxy. The proxy must return a Proxy-Authenticate header field containing a challenge applicable to the proxy for the requested resource. The client may repeat the request with a suitable Proxy-Authorization header field. HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication".
Related tags
See also RFC2616.